Add Maven API documentation

doc/api/packages/maven.md

+---
+stage: Package
+group: Package
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Maven API
+
+This is the API documentation for [Maven Packages](../../user/packages/maven_repository/index.md).
+
+WARNING:
+This API is used by the [Maven package manager client](https://maven.apache.org/)
+and is generally not meant for manual consumption.
+
+For instructions on how to upload and install Maven packages from the GitLab
+package registry, see the [Maven package registry documentation](../../user/packages/maven_repository/index.md).
+
+NOTE:
+These endpoints do not adhere to the standard API authentication methods.
+See [Maven package registry documentation](../../user/packages/maven_repository/index.md)
+for details on which headers and token types are supported.
+
+## Download a package file at the instance-level
+
+> Introduced in GitLab 11.6.
+
+Download a Maven package file:
+
+```plaintext
+GET packages/maven/*path/:file_name
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `path`       | string | yes | The Maven package path, in the format `<groupId>/<artifactId>/<version>`. Replace any `.` in the `groupId` with `/`. |
+| `file_name`  | string | yes | The name of the Maven package file. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.jar"
+```
+
+To write the output to file:
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.jar" >> mypkg-1.0-SNAPSHOT.jar
+```
+
+This writes the downloaded file to `mypkg-1.0-SNAPSHOT.jar` in the current directory.
+
+## Download a package file at the group-level
+
+> Introduced in GitLab 11.7.
+
+Download a Maven package file:
+
+```plaintext
+GET groups/:id/-/packages/maven/*path/:file_name
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `path`       | string | yes | The Maven package path, in the format `<groupId>/<artifactId>/<version>`. Replace any `.` in the `groupId` with `/`. |
+| `file_name`  | string | yes | The name of the Maven package file. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/groups/1/-/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.jar"
+```
+
+To write the output to file:
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/groups/1/-/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.jar" >> mypkg-1.0-SNAPSHOT.jar
+```
+
+This writes the downloaded file to `mypkg-1.0-SNAPSHOT.jar` in the current directory.
+
+## Download a package file at the project-level
+
+> Introduced in GitLab 11.3.
+
+Download a Maven package file:
+
+```plaintext
+GET projects/:id/packages/maven/*path/:file_name
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `path`       | string | yes | The Maven package path, in the format `<groupId>/<artifactId>/<version>`. Replace any `.` in the `groupId` with `/`. |
+| `file_name`  | string | yes | The name of the Maven package file. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.jar"
+```
+
+To write the output to file:
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.jar" >> mypkg-1.0-SNAPSHOT.jar
+```
+
+This writes the downloaded file to `mypkg-1.0-SNAPSHOT.jar` in the current directory.
+
+## Upload a package file
+
+> Introduced in GitLab 11.3.
+
+Upload a Maven package file:
+
+```plaintext
+PUT projects/:id/packages/maven/*path/:file_name
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `path`       | string | yes | The Maven package path, in the format `<groupId>/<artifactId>/<version>`. Replace any `.` in the `groupId` with `/`. |
+| `file_name`  | string | yes | The name of the Maven package file. |
+
+```shell
+curl --request PUT \
+     --upload-file path/to/mypkg-1.0-SNAPSHOT.pom \
+     --header "Private-Token: <personal_access_token>" \
+     "https://gitlab.example.com/api/v4/projects/1/packages/maven/foo/bar/baz/mypkg-1.0-SNAPSHOT.pom"
+```

doc/user/packages/maven_repository/index.md

@@ -12,6 +12,9 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 Publish [Maven](https://maven.apache.org) artifacts in your project's Package Registry.
 Then, install the packages whenever you need to use them as a dependency.
 
+For documentation of the specific API endpoints that the Maven package manager
+client uses, see the [Maven API documentation](../../../api/packages/maven.md).
+
 ## Build a Maven package
 
 This section explains how to install Maven and build a package.